The oncogene c-myc is abnormally expressed in several forms of human cancer, notably Burkitt's lymphoma, colorectal carcinoma and promyelocytic leukemia. It is known that this gene is highly expressed in these cells, due to its amplification or chromosomal translocation, but how this results in abnormal cellular growth has not been clear. Recent work in this laboratory indicated that c- myc protein participates in DNA replication of human cells. Thus, overexpression of this gene product would confer a proliferative advantage to the cell, by making it less dependent on external growth stimuli. It is proposed to exploit this finding to determine more precisely the step or steps in DNA replication which are dependent on the c-myc gene product. The DNA replication complex will be dissociated and reconstituted following an exposure to affinity-purified polyclonal and monoclonal antibodies to c-myc (alpha-myc). Re-addition of individual components of the replication machinery will identify cellular proteins that react with alpha-myc. Studies with replicating molecules of SV 40 virus will provide conditions analogous to an amplified mammalian replication unit, and thus permit a closer analysis of the stepwise maturation of nascent DNA. The role of oncogene c-fos, in cells which express this oncogene, in DNA replication or maintenance of chromatin structure will be addressed by sequential immunocytochemical and in situ hybridization studies, and related to DNA replicative activity. These studies should provide a beginning for the clarification of the physiological functions of oncogenes encoding nuclear phosphoproteins.